This invention relates generally to the field of magnetic resonance imaging (MRI) which utilizes and is based upon the phenomenon of nuclear magnetic resonance (NMR) to generate visual depictions of spatial distributions of NMR nuclei interior to a structure such as the living human body. This invention is particularly useful in making quadrature detection (QD) coil assemblies for use in MRI of the human head--especially when using lower strength polarizing magnetic fields (and thus correspondingly lower NMR RF frequencies).
Magnetic resonance imaging is by now well-known and in widespread commercial use. In general, a strong, nominally static, nominally uniform, polarizing magnetic field tends to align significant proportions of the body nuclei which have a net magnetic moment. By suitably nutating these populations of nuclei using NMR phenomena (e.g., with a succession of RF nutation pulses and coordinated magnetic gradient pulses), NMR RF signals are elicited from these nuclei. Such NMR RF signals include spatially encoded information that can later be processed to produce a visual map of the NMR nuclei density along desired surfaces passing through the human body.
One of the factors critical to successful MRI is efficient RF coupling to the weak NMR RF signal responses. One known technique for enhancing the signal-to-noise ratio, of such detected NMR RF signals involves the use of so-called quadrature detection RF receiving coils. In general, the magnetic field associated with NMR RF responses is orthogonal to the static polarizing magnetic field. By arranging to have two independent RF reception coils also disposed orthogonally to one another, one can subsequently combine these two independent channels of signal reception so as to provide an increased signal-to noise-ratio in the net received signal.
For higher strength polarizing fields (and thus correspondingly higher NMR RF frequencies) there are various QD coil assemblies already known in the prior art. For lower strength magnetic fields (and/or for polarizing fields that are disposed vertically rather than horizontally), solenoidal RF receiving coils are sometimes more appropriately utilized. However, it has heretofore been quite difficult to find a solenoidal RF coil structure that can be conveniently used for quadrature detection while at the same time permitting convenient ingress and egress of relevant human body portions (e.g., the head) into the multi-coil structure.
RF coils used in MRI typically utilize a combination of inductance and capacitance to resonate at the NMR frequency of the nuclear species to be imaged. For the most prevalent proton imaging, this frequency is typically in the range of 2 to 70 megahertz, depending upon the strength of the static polarizing magnetic field. Above about 10 megahertz, there are several known designs that may be used for MRI RF coils. However, at lower frequencies, most of these designs become impractical. One reason for such impracticality is that the resonant frequency of a coil changes inversely with respect to the square root of the product of its inductance and capacitance. Therefore, as the resonant frequency is lowered, the product of inductance and capacitance must increase as the inverse square of the frequency. Most previous MRI RF coil designs do not have sufficient inductance to make efficient coils below about 5 megahertz.
The most efficient coil design for low field MRI is probably the simplest, namely, the solenoid. The solenoid has several advantages, most notably, its relatively high inductance and good field uniformity. However, adapting the solenoid to quadrature detection for MRI is quite difficult.
For example, to realize a quadrature detection MRI RF coil, one needs two separate resonant coils, each of which produces a magnetic field disposed perpendicularly with respect to the static polarizing magnetic field. In addition, the fields of the two resonant coils must be perpendicular to each other--i.e., the coils should not be inter-coupled since this will make tuning difficult and/or result in degradation of signal-to noise-ratio. As those skilled in the art will appreciate, decoupling two high Q (i.e., quality factor) resonators which are in close proximity can be an extremely difficult problem.
Consider, for example, the coil assembly of FIG. 1. Here, the assembly includes two separate solenoidal resonators; an inner-coil 10 ( shown in dotted lines) contained inside an outer coil 12 (shown in solid lines). As drawn in FIG. 1, the longitudinal axis of the inner-coil coincides with the x-coordinate axis while the longitudinal axis of the outer coil coincides with the y-coordinate axis. The ends of the first and last turns of these coils are connected with a return loop RF connection including a resonating capacitor. Since the solenoidal coils 10, 12 have their longitudinal axes mutually perpendicular, it might appear at first glance that such an arrangement would satisfy the requirements for a possible quadrature detection (QD) MRI RF coil arrangement (forgetting for a moment about the problem of ingress and egress of human anatomy). However, an experiment with a pair of such solenoidal RF coils as depicted in FIG. 1 will show a relatively strong inter-coil coupling. Of course, this is contrary to desired QD MRI RF coil design criteria.
Detailed study of the inter-coil coupling that is experienced with an arrangement such as that shown in FIG. 1 has revealed that such coupling is related, among other things, to the fact that each turn of the solenoids 10 and 12 is pitched at an angle with respect to its longitudinal axis. The inter-coil coupling of the assembly shown in FIG. 1 also is affected by positioning of the current return path (i.e., the conductor and serial resonating capacitor connecting the ends of the solenoid). While it is true that the magnetic field associated with inner-coil 10 is primarily along the x-axis, because the turns are pitched ( and because of the positioning of the current return path--i.e., contained in the x-z plane, the z-axis protruding from the plane of FIG. 1, there will be an additional equivalent current loop in the x-z plane which produces a field oriented in the y-axis direction. Therefore, such an equivalent current loop will couple directly to the main field of the second coil, producing disadvantageous inter-coil coupling. Similarly, as depicted in FIG. 1, the outer coil 12 will produce some field along the x-axis direction which couples directly to the main field of the inner-coil 10, thus further compounding the inter-coil coupling problem.
Of course, decoupling may still be achieved between the inner-coil 10 and the outer coil 12 in FIG. 1 by adjusting the orientation, i.e., angle, between the longitudinal axes of the two coils. At some angle other that 90.degree., the net fields produced by the two coils will be orthogonal and the coils will be decoupled from one another. Unfortunately, such decoupling by angular repositioning will be extremely sensitive to the angle adjustment and to other minor variations in coil construction.